1. Field of the Invention
The present invention relates to an optical information recording device and method for generating interference on an information recording layer of a recording medium by irradiating information light holding information and reference light to the recording medium using an object lens such as to record information using these interference patterns, and an optical information reproduction device and method for making reference light interfere with interference patterns recorded on an information recording layer of a recording medium by irradiating reference light to the recording medium by an object lens such as to generate reproduction light holding information and reproduce information.
2. Description of the Related Art
Holographic recording for recording information to a recording medium, utilizing holography, is generally performed by superimposing light, holding image information, and reference light within the recording medium and writing the interference patterns formed at this time to the recording medium. And when reproducing the information recorded in the recording medium, the image information is reproduced by diffraction due to the interference patterns by irradiating the reference light to this recording medium.
In recent years, volume holography, digital volume holography in particular, has been developed on a practical level and is receiving attention for ultra-high-density optical recording. Volume holography is a system for writing interference patterns three-dimensionally, actively using the thickness direction of a recording medium as well, and can enhance diffraction efficiency by increasing thickness and increase recording capacity by using multiplex recording. In addition, digital volume holography is a computer-oriented holographic recording system which limits image information to be recorded to binarized digital patterns, while implementing the same recording media and recording system as volume holography. In this digital volume holography, graphic information such as analog illustration, for example, is temporarily digitalized and developed into binary digital pattern information which is recorded as image information. When reproducing, this image information is returned to the original graphic information and displayed by reading and decoding this information. Through this, the original information can be reproduced with extreme accuracy by performing differential detection or encoding binary data and correcting errors, even if the signal-to-noise ratio (SN ratio) is somewhat poor when reproducing.
Incidentally, a common recording device for recording information to a disk-shaped recording medium by utilizing light comprises an optical head for irradiating information recording light to the recording medium. Furthermore, this recording device records information to the recording medium by irradiating information recording light from the optical head to the recording medium, while rotating the recording medium. In addition, semiconductor laser is generally used as a light source for generating information recording light in this recording device.
As in the foregoing common recording device, successively-recording information in a plurality of information recording areas within the recording medium by irradiating information light and reference light to the recording medium, while rotating the recording medium, is possible in holographic recording, as well. In this case, it is also preferable to use a practical semiconductor laser as the light source for information light and reference light in holographic recording, as in the foregoing common recording device.
An optical system for recording/reproduction provided within a light head 40 is described below with reference to a cross-sectional view of the light head in Japan Unexamined Patent Application 2002-183975 (Patent Reference 1), shown in FIG. 16.
The light head 40 has a head body 41 which houses each of the constituents described hereafter. A semiconductor laser 43 is fixed onto the bottom of this head body 41 with a support 42 in between, and a reflective phase spatial light modulator 44 and an optical detector 45 are fixed onto the bottom of this head body 41, as well. A microlens array 46 is mounted onto the acceptance surface of the optical detector 45. In addition, the prism block 48 is provided above the phase spatial light modulator 44 and the optical detector 45, within the head body 41. A collimator lens 47 is provided in the vicinity of the end of the prism block 48 on the semiconductor laser 43-side. In addition, an opening is formed on the side of the head body 41 facing a recording medium 1, and an object lens 11 is provided at this opening. A ¼ wavelength plate 49 is provided between this object lens 11 and the prism block 48.
The phase spatial light modulator 44 has numerous pixels aligned in a lattice and can set the phase of emission light for every pixel to either one of two values which differ from each other by π radians such as to spatially modulate the phase of the light. Furthermore, the phase spatial light modulator 44 is configured to rotate the polarizing direction of the emission light by 90° to the polarization direction of incident light. A reflective liquid crystal element, for example, can be used as the phase spatial light modulator 44.
The optical detector 45 has numerous pixels aligned in a lattice and can detect the intensity of light received by each pixel. In addition, microlens array 46 has a plurality of microlenses, each of which is placed in a position facing the acceptance surface of each pixel of the optical detector 45.
A CCD-type solid-state image sensing device and MOS-type solid-state image sensing device can be used as the optical detector 45. In addition, a smart optical sensor, wherein a MOS-type solid-state image sensing device and a signal processing circuit are integrated on one chip (for example, refer to Reference “O plus E, September 1996, No. 202, Pages 93 to 99”), can be used as the optical detector 45. This smart optical sensor has a large transfer rate and a high-speed operation function and enables high-speed reproduction, for example, enabling reproduction at a transfer rate of G (giga) bit/second order.
The prism block 48 has a polarized beam splitter surface 48a and a reflective surface 48b. The polarized beam splitter surface 48a is positioned nearer to the collimator lens 47 than the reflective surface 48b is. Both the polarized beam splitter surface 48a and the reflective surface 48b are positioned such that their normal directions have oblique angles of 45° to the optical axis direction of the collimator lens 47 and they are positioned such as to be parallel to one another.
The phase spatial light modulator 44 is positioned under the polarized beam splitter surface 48a and the optical detector 45 is positioned under the reflective surface 48b. In addition, the ¼ wavelength plate 49 and the object lens 11 are positioned over the polarized beam splitter surface 48a. Hologram lenses can be used as the collimator lens 47 and the object lens 11. The polarized beam splitter surface 48a of the prism block 48 separates the optical path for the information light, the recording reference light and the reproduction reference light before passing through the ¼ wavelength plate 49 and that for the return light from the recording medium 1 after passing through the ¼ wavelength plate 49, using the differences in their polarizing directions.
Next, the operation of the optical system for recording/reproduction at the time of recording of information is briefly described below.
A semiconductor laser emits coherent polarized light S. The polarized light S is linearly polarized light with a polarizing direction perpendicular to the entrance plane. Polarized light P, described hereafter, is linearly polarized light with a polarizing direction parallel to the entrance plane.
The laser light of the polarized light S emitted from the semiconductor laser 43 is formed into parallel light by the collimator lens 47, irradiated onto the polarized beam splitter surface 48a of the prism block 48, and is reflected by this polarized beam splitter surface 48a such as to enter the phase spatial light modulator 44.
In a conventional optical information recording/reproduction device, the information light and the recording reference light are generated by the phase spatial light modulator 44. The phase spatial light modulator 44 is configured such that a coherent parallel light with constant phase and intensity is irradiated thereto. In the phase spatial light modulator 44, when recording information, the display area is equally divided into two parts, wherein information light is generated with its phase spatially modulated in one part of the area by selecting the phase of the emission light for every pixel, depending on information to be recorded, and recording reference light is generated in the other part by equalizing or spatially modulating the phase of the emission light for every pixel.
The information light and the recording reference light emitted from the phase spatial light modulator 44, being polarized lights P, pass through the polarized beam splitter surface 48a of the prism block 48 and the ¼ wavelength plate 49 and become circular polarized lights. These information light and recording reference light are irradiated onto the recording medium 1 after being converged by the object lens 11. These information light and recording reference light pass through an information recording layer 3, are converged on the boundary surface between an air gap layer 4 and a reflective layer 5 such that the radii of these lights become minimal and are reflected by the reflective layer 5. After being reflected by the reflective layer 5, the information light and the recording reference light are diffused and pass through the information recording layer 3 again. If the output of the semiconductor laser 43 is set to high output for recording, interference patterns, generated by interference between the information light and the recording reference light, are recorded to the information recording layer 3.
The return light from the recording medium 1 is formed into parallel light by the object lens 11 and becomes the light of polarized light S after passing through the ¼ wavelength plate 49. This return light is reflected by the polarized beam splitter surface 48a of the prism block 48, further reflected by the reflective plane 48b, and enters the optical detector 45 after passing through the microlens array 46.
When recording information, during the period when the light beam from the object lens 11 passes through an address servo area 6 of the recording medium 1, the output of the semiconductor laser 43 is set to low output for reproduction. At the same time, the phase spatial light modulator 44 emits light with the same phase for every pixel, without modulating the phase. Based on the output of the optical detector 45 during this period, address servo information, such as the basic clock, the address information, the focus error signal and the tracking error information, can be obtained
Next, the operation of the optical system for recording/reproduction at the time of reproducing information is described.
When reproducing information, the output of the semiconductor laser 43 is set to low output for reproduction. The laser light of polarized light S emitted from the semiconductor laser 43 is formed into parallel light by the collimator lens 47, enters the polarized beam splitter surface 48a of the prism block 48, and is reflected by this polarized beam splitter surface 48a such as to enter the phase spatial light modulator 44. The emission light from the phase spatial light modulator 44 becomes a reproduction reference light wherein its phase is equalized or spatially modulated for every pixel. In addition, the emission light from the phase spatial light modulator 44 becomes the light of polarized light P after the polarizing direction is rotated 90°.
The reproduction reference light emitted from the phase spatial light modulator 44, being polarized light P, passes through the polarized beam splitter surface 48a of the prism block 48 and the ¼ wavelength plate 49 and becomes circular polarized light. This reproduction reference light is irradiated onto the recording medium 1 after being converged by the object lens 11. This reproduction reference light passes through the information recording layer 3, is converged on the boundary surface between the air gap layer 4 and the reflective layer 5 such that the radius of this light becomes minimal and reflected by the reflective layer 5. After being reflected by the reflective layer 5, the reproduction reference light is diffused and passes through the information recording layer 3 again. The reproduction light is generated from the information recording layer 3 by the reproduction reference light.
The return light from the recording medium 1 includes the reproduction light and the reproduction reference light. This return light is formed into parallel light by the object lens 11 and becomes the light of polarized light S after passing through the ¼ wavelength plate 49. This return light is reflected by the polarized beam splitter surface 48a of the prism block 48, further reflected by the reflective plane 48b, and enters the optical detector 45 after passing through the microlens array 46. Information recorded in the recording medium 1 can be reproduced, based on the output of this optical detector 45.
When reproducing information, based on the output of the optical detector 45 during the period when the light beam from the object lens 11 passes through the address servo area 6 of the recording medium 1, address servo information, such as the basic clock, the address information, the focus error signal and the tracking error information, can be obtained.
The phase spatial light modulator 44 can be that which does not rotate the polarizing direction of the light. In this case, the polarized beam splitter surface 48a of the prism block 48 in FIG. 16 must be changed to a half-reflective surface. Alternatively, a ¼ wavelength plate can be provided between the prism block 48 and the phase spatial light modulator 44, the light of the polarized light S from the prism block 48 can be changed to circular polarized light by the ¼ wavelength plate and irradiated onto the phase spatial light modulator 44, the circular polarized light from the phase spatial light modulator 44 can be changed to the light of the polarized light P and passed through the polarized beam splitter surface 48a. In addition, the phase spatial light modulator, which is capable of setting the phase of the emission light for every pixel to either one of three or more values, is not limited to that utilizing liquid crystal elements and, for example, can be configured to enable adjustment of the position of the reflective surface for every pixel, with regards to the traveling direction of the incident light, utilizing micromirrors.
In a light information recording/reproduction device such as that shown in U.S. Pat. No. 6,108,110 (Patent Reference 2), information light is generated by a spatial light modulator which has a plurality of pixels, reference light is generated by a light diffusion device placed in the vicinity of the spatial light modulator, and interference is generated on the information recording layer of a recording medium.